1. Field of the Invention
This invention relates to an arrangement for measuring a deviation from its line of a movable web of foil.
Such measuring arrangements are advantageously used for cutting an embossed web of plastics foil on re-winding cutting machines, in order to sense precisely the lateral position of the web of foil.
2. Description of the Prior Art
Optical line-of-movement or tracking sensors are known which, during unwinding and re-winding of a web of foil, continuously measure the lateral deviation of the web from a prescribed central position by means of optical sensing of an edge of the web. The tracking sensor comprises a light source with an optical system acting as a transmitter on one side of the web of foil, and a detector acting as a receiver on the other side. It determines the position of the edge of the web, which masks a light beam from the transmitter to a greater or lesser degree. The tracking sensor transmits sensed signals to a measuring device which produces a control signal for correcting the deviation, the control signal being supplied to guide rollers. The web of foil is returned to its central position by means of the guide rollers. A disadvantage with this arrangement is the sensitivity of the tracking sensor to flaws in the edge of the web of foil and to the transparency of the foil.
On the other hand, tracking sensors are known which follow a marking track produced by graphic means on the web of foil. These optical tracking sensors comprise a lighting device acting as the transmitter and a receiver for reflected light, which is arranged on the same side of the web. They detect the marking track by virtue of differences in reflectivity. Such tracking sensors are sensitive to variations in the intensity of the lighting and to alterations in the reflectivity of the web of foil and the marking track.